1. Field of the Invention
The present invention relates to a controller for an automatic transmission having a torque converter with lockup clutch. More particularly, the invention relates to a controller providing control to switch target slip ratios of the lockup clutch, according to a running condition of a vehicle.
2. Description of the Related Art
The automatic transmission for use in vehicle is provided with the lockup clutch which is incorporated in a torque converter and adapted to mechanically interconnect an output shaft of an engine and an input shaft of the automatic transmission. It has been a common practice to keep engine speed low by engaging the lockup clutch under given conditions so as to achieve better fuel economy.
The lockup clutch is normally placed in engagement when the automatic transmission is shifted into a predetermined gear. When fully engaged, however, the lockup clutch is incapable of absorbing shock transmitted between the above input shaft and output shaft. It is therefore a common practice to provide a slip control in which the lockup clutch is placed in slipping engagement rather than in full engagement depending upon driving status of the vehicle. According to such a slip control, a target slip ratio for the lockup clutch according to a driving status is previously stored in correspondence to each of the gears, so that the engaging capacity of the lockup clutch is controlled (through feedback) in a manner such that an actual slip ratio of the lockup clutch conforms to the target slip ratio.
In recent design contemplated to achieve even better fuel economy, a working range of the lockup clutch is expanded by setting an even lower temperature for automatic transmission fluid (ATF) at the start of control of the lockup clutch. In this case, however, a friction modifier present in the automatic transmission fluid is less activated so that the lockup clutch is deteriorated in friction characteristic. This makes the above-described feedback control instable, which may lead to engine hunting and associated vibration of vehicle body.
More specifically, a wet type clutch generally has a friction characteristic that if the clutch has a constant surface pressure, transmission torque (T) is increased with the increase in differential rotation (V) (characteristic graphically represented by an ever increasing curve). To engage the clutch, therefore, hydraulic pressure may be applied to the clutch with a predetermined variation (gradient). In a state where the automatic transmission fluid is at low temperatures, however, the friction modifier is not fully activated and hence, the above-described differential rotation-transmission torque characteristic (V-T characteristic) of the clutch is graphically represented by a flat line or ever decreasing curve. If the application of hydraulic pressure is continued in this state, the clutch is prone to abrupt change in transmission torque so as to be lowered in followability to a target value of the feedback control. This results in an increased incidence of the engine hunting.
According to automatic transmissions disclosed in JP-A-Hei 3-30023 and JP-B-3585207, the feedback control of the engaging capacity of the lockup clutch is performed in a manner such that feedback gain is switched according to the temperature of the automatic transmission fluid. However, only the control by switching the feedback gain according to the automatic; transmission fluid temperature cannot fully improve the lockup clutch in the followability to the target value of the feedback control. A final solution to the above problem is not achieved.